Antioxidants formed from polymerizing alkylated diphenylamines and alkylated phenyl-α-naphthyl amines are widely used in many industrial, automotive, and aviation lubricants, greases, and fluids to improve their oxidative stability. The polymerization reaction is based on oxidizing alkylated diphenylamines and alkylated phenyl-α-naphthyl amines to form a mixture of homo- and cross-oligomers of these compounds. In some prior art processes, the polymerization occurs in the presence of air or oxygen at high temperature (e.g., 150° C. to 300° C. (for example, U.S. Pat. No. 3,509,214).
In other processes, chemical oxidizing agents such as organic peroxides, potassium permanganate, and the like can be used to promote the oxidizing reaction (for example, U.S. Pat. Nos. 3,509,214 and 6,426,324). There are certain safety issues, however, associated with storing and handling organic peroxides.
There is a continuing need, therefore, to provide improved processes and antioxidants.